The communications links across which computers—or parts of computers—talk to one another may be either serial or parallel. A parallel link transmits several streams of data (perhaps representing particular bits of a stream of bytes) along multiple channels (wires, printed circuit tracks, optical fibers, etc.), while a serial link transmits a single stream of data over only two wires (a positive and complementary signal). At first sight it would seem that a serial link must be inferior to a parallel one, because it can transmit less data on each clock tick. However, it is often the case that serial links can be clocked considerably faster than parallel links and can achieve a higher data rate. A number of factors allow serial links to be clocked at a greater rate. First, clock skew between different channels is not an issue (for un-clocked serial links). Second, a serial connection requires fewer interconnecting cables (e.g. wires/fibers) and hence occupies less space allowing for better isolation of the channel from its surroundings. Finally, crosstalk is less of an issue because there are fewer conductors in proximity. In many cases, serial links are a better option because they are less expensive to implement. Many integrated circuits (ICs) have serial interfaces, as opposed to parallel ones, so that they have fewer pins and are therefore more economical.
Despite their advantages, serial links tend to use more power than parallel links. In particular, when transmitting or receiving data, a serial link changes states very rapidly (called toggling). A small amount of power is consumed by each state change, thereby adding up to large power consumption over time. Serial links are also typically terminated on each end by a termination resistor and bias resistors. Without termination resistors, reflections of fast driver edges can cause multiple data edges that can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. The bias resistors bias the lines apart when the lines are not being driven. Without biasing resistors, the signal falls to zero (where electrical noise sensitivity is greatest) when no data is being transmitted. Both termination and bias resistors are therefore necessary; however, the additional resistance causes the link to consume a constant amount of power to keep the link alive.
Consumers demand higher and higher speeds from electronic devices, but the higher the speed of the device the more power the device consumes. This is particularly a problem for mobile devices that have limited power available through on-board batteries. To provide the most benefit, these devices must have a long battery life while still providing consumers with a high degree of functionality. Previous attempts at power reduction have attempted to power down the device or place it into a lower power state when it is not in use, and then rapidly return it to full power when the user of the device wants to perform a function. However, this technique is not effective when a device is frequently in use and can still result in significant power being consumed.